During semiconductor chip manufacture, unintentional formation of surface defects on the semiconductor wafers is undesirable. Watermarks are one such defect unintentionally formed during wafer processing.
Generally, a semiconductor wafer has a silicon surface with a thin layer of oxide on the silicon. The oxide layer can be removed by subjecting the semiconductor wafer to a wet chemical processing step, e.g., a dilute hydrofluoric acid dip. After such processing, the semiconductor wafer is normally rinsed with deionized water to remove the chemicals used in the chemical processing step. However, silicon at the surface of the wafer can undergo dissolution upon contact with the deionized water to form the reaction product Si(OH).sub.4 in solution. When the rinsed semiconductor wafer is subsequently dried the silicon in solution precipitates back onto the surface of the semiconductor wafer, producing watermarks on the surface of the wafer.
The etching of silicon in the presence of dissolved oxygen in water is explained in Eisenberg et al., Surface Chemical Cleaning and Passivation for Semiconductor Processing, Materials Research Soc. Symp. Proc. No. 315, Pittsburgh, pp. 485-490 (1993) as follows: First, the Si--Si backbond is reacted with oxygen. Second, the silicon atoms attain a positive charge due to the electronegativity of the oxygen which initiates the reaction of OH.sup.- with the Si--Si backbond. The third step involves the release of Si.sub.x O.sub.y H.sub.z into the solution and the termination of the silicon bond with hydrogen. Unintentional etching of silicon and the formation of watermarks is a potential problem whenever an aqueous solution comes into contact with a bare or unprotected silicon surface.
It would be desirable to provide an easily implemented method of inhibiting silicon dissolution to reduce the formation of watermarks during wafer processing and thereby increase production yield of semiconductor chips.